Tabanus Ommatidia

Black horse fly (Tabanus atratus)

The compound eyes of insects are composed of hundreds to thousands of ommatidia, each a bundle of photoreceptor cells with a hexagonal facet that receives light independently and together forms a composite image. The more ommatidia, the greater the visual resolution, allowing predatory flying insects such as dragonflies to perform precise aerial acrobatics to intercept prey. The eyes of tabanid flies are also striking because of their striped or mosaic patterns. Below is a macro shot of a 2.5 cm long Tabanus sulcifrons I found today in Missouri. I’m not sure about the functional significance of these striking eye patterns, but one paper suggests the different bands may act like bifocals— these corneal filters reduce transmission of certain wavelengths of ambient light. Along these lines, the authors go on to suggest that the filters might serve to enhance contrast in bright light conditions between black and white on the legs and body, which may be signals relevant to courtship in some families of flies (e.g. long-legged flies; Dolichopodidae) and possibly the tabanids.

Tabanus sulficrons

Following up from my zebra post, a really interesting theory for the long-standing question of why zebras have stripes involves the visual system of biting flies such as this tabanid. Experiments have repeatedly shown that biting flies land successfully on striped targets at a much lower frequency than on non-striped ones. Yep, horses were dressed up in striped suits! and flies bounced off them clumsily. However, rates of approach and circling by tabanid flies are no different between striped and non-striped treatments. The underlying mechanisms by which stripes deter biting flies at close range is still up for debate, but one of the leading hypotheses is that the contrast in polarization between the light and dark stripes disrupts polarotaxis (tabanids orient themselves to polarized light) needed to achieve landing. But how can biting flies be a strong enough selective pressure for such a dramatic suit of colors? It turns out that biting flies such as tse-tse flies and tabanids carry all sorts of pathogens that can be fatal to equids, such as trypanosomiasis and infectious anemia. With constant exposure to the blood-sucking insects under the hot African sun, shakes of the body and tail can only do so much…


09/18/2019: Large horseflies like this Tabanus atratus are easy to find along the streets here in Missouri. Many of them land on hot sidewalks and metal railings to heat up, but end up surpassing their critical thermal max— losing motor function and becoming easy pickings for a bird predator (and myself too). All Tabanus found weakened after lying exposed on the concrete; photographed after capture under controlled conditions [5].

This evening I am reading about light signal reception. So, following up from my post last month about horsefly vision… why are invertebrate rhabdomeric photoreceptors more sensitive to polarized light than our own ciliary receptors? In ciliary receptors (rods and cones), stacks of flat round discs contain pigment molecules that are all arranged perpendicularly to the direction of light— imagine a stack of cookies with random bar-shaped sprinkles on top. These receptors are unable to distinguish rays of different polarizations of downwelling light (axial rays) because all vectors are absorbed equally by the perpendicularly-oriented molecules, while for vectors coming in at an angle (transverse rays) they can only absorb one plane of horizontally-polarized light. In contrast, rhabdomeric receptors are composed of adjacent cylindrical microtubules that can contain all orientations of pigment molecules in 3D space—  imagine a group of churros with sprinkles along all the sides. So, depending on the alignment of those molecules (or sprinkles), some vectors of light will be preferentially absorbed. In the compound eyes of insects with thousands of ommatidia, some locations may be specialized in having all pigment molecules oriented in the same direction, resulting in improved detection of plane-polarized light. I found this concept really difficult to explain concisely, but see the diagram below if you are interested!

Diagram from “Principles of Animal Communication“, by Bradbury & Vehrencamp

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